Investigation on the effect of Tb(dbm)3phen on the luminescent properties of Eu(dbm)3phen-containing mesoporous silica nanoparticles

https://doi.org/10.1016/j.matchemphys.2013.07.054Get rights and content

Highlights

  • Detailed study of Eu(dbm)3phen-doped mesoporous silica nanoparticles luminescence.

  • Inclusion of up to 23 wt% of Eu(dbm)3phen without concentration quenching.

  • Detailed study of the role of the Tb(dbm)3phen co-dopant.

  • Co-doping effective for Eu3+(dbm)3phen loadings lower than about 10 wt%.

Abstract

Eu(dbm)3phen and Tb(dbm)3phen complexes (tris(dibenzoylmethane) mono(1,10-phenantroline) Ln(III)) were impregnated in ordered mesoporous silica nanoparticles (MSNs) with an average size of 50–70 nm and a pore diameter centred at 2.8 nm, with the aim of increasing the luminescence by avoiding concentration quenching and having mainly in mind the application as down-shifter for multi-crystalline solar cells. The morphological, structural, textural and luminescent properties of the synthesized samples were characterized by N2 adsorption–desorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), UV–visible spectroscopy and photoluminescence measurements. It is demonstrated that inclusion in the MSNs allows one to use much higher loadings (23 wt%) of the Eu-complex than in other matrices, and that co-doping with Tb(dbm)3phen improves luminescence for samples with Eu(dbm)3phen content lower than about 10 wt%. Results are interpreted by using a simple sphere of action model adapted to the case of a pore-limited system.

Graphical abstract

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Sensitization of the antenna effect (down-conversion of UV radiation to red light) by the presence of Tb(dbm)3phen in the cavities of mesoporous silica nanoparticles containing Eu(dbm)3phen.

Introduction

Lanthanide organic complexes have been widely studied for their interesting absorption and emission features and have been suggested for applications in many different fields such as probes for biological systems [1], [2], LED [3], OLEDs [4], laser materials [5], or spectral converters for solar cells [6], [7], [8], [9], [10], [11].

The present work was carried out having mainly the latter application in mind, where the developed material needs to meet special requirements, like transparency, broad band absorption, long-term stability. In fact, harnessing those regions of the solar spectrum which are only weakly, or not at all, converted into electricity by multicrystalline silicon solar cells is highly desirable, in order to increase efficiency and reach grid-parity. Down-shifters, which convert UV radiation into visible or near infra-red light, may serve this goal [8]; these materials need to efficiently absorb a broad spectral range between 300 nm and 450 nm and re-emit with a large Stokes shift in the region where the solar cells show a significantly better response. Lanthanide ions have sharp emission profiles with high internal efficiency, but their absorption is very small and it takes place only on very specific wavelengths; this may be appropriate for applications where excitation can be made by lasers, but not for solar spectrum conversion. However, if the lanthanide ion is conjugated with suitable organic ligands having an electronic structure which matches that of the lanthanide, the desired optical properties for this application may be obtained; in fact, the ligand acts as an antenna, efficiently absorbing in a broad spectral region (typically in the UV) and transferring this energy to the rare earth ion, which then emits in the visible. At the same time, in order to obtain a good quantum yield, the emitted radiation has not to be quenched by the chemical environment, or the matrix in which the ions are included, and/or by concentration quenching. Furthermore, the organic ligand may suffer from UV degradation or thermal instability.

In order to address such problems two synergic approaches have been tested in this paper. The first is that of hosting the complex molecules within the pores of properly synthesized mesoporous silica nanoparticles. The pores were chosen of suitable size in order to “dilute” the emitting ions, thus avoiding concentration quenching and at the same time protecting them from the environment. Such an approach has shown to improve luminescence in similar cases [12], [13], [14], [15]. Since the final goal is to obtain a layer transparent to visible light, the mesoporous silica must be in the form of nanoparticles to avoid light scattering; such nanoparticles can be then dispersed in the encapsulating layer of the solar cell. We show that this approach is able to host up to 23 wt% of a lanthanide complex before concentration quenching takes place, which means some orders of magnitude more than in solution.

The second approach is that of co-doping with two different lanthanide complexes, where not only the antenna, but also the second lanthanide ion can act as sensitizer for the first ion. To this purpose, the Tb3+/Eu3+ pair is well known, where Tb ions can efficiently transfer energy to Eu ions increasing the latter's emission in the red spectral region. In this situation UV light is absorbed by the ligand and then transferred to Eu ions, either directly or indirectly mediated by Tb. Optimizing the ratio between Tb- and Eu-complexes within the pores of silica nanoparticles, it is possible to avoid concentration quenching [16] and obtain high red emission intensity.

In the present paper the behaviour of silica mesoporous nanoparticles with different loadings of Eu-complex is investigated in details and the effect of adding a second lanthanide complex is studied.

Section snippets

Materials

Tetraethyl orthosilicate (TEOS, Aldrich, 98%), hexadecyl-trimethylammonium bromide (CTAB, Aldrich), europium trichloride hexahydrate (EuCl3·6H2O, Aldrich, 99.9%), terbium trichloride hexahydrate (TbCl3·6H2O, Aldrich, 99.9%), ammonium hydroxide solution (Fluka, 28 wt% in water), absolute ethanol (99.8%, Carlo Erba) were all used as received. Eu(dbm)3phen and Tb(dbm)3phen complexes (tris(dibenzoylmethane) mono(1,10-phenantroline) Ln(III)) were synthesized according to the procedure reported by

The complex

Fig. 1 shows the molecular sketches of the two complexes used in this paper. They are formed by one phenanthroline and three dibenzoylmethane ligands connected by dative bonds to the lanthanoid ion, Eu or Tb, respectively.

Fig. 2 shows absorption, excitation and emission spectra of the Eu(dbm)3phen complex in solution. Absorption takes place on two bands: one, centred at 270 nm, is attributed to the absorption by the phenanthroline ligand [16], and is of no interest for terrestrial solar

Conclusions

The inclusion of Eu(dbm)3Phen inside mesoporous silica nanoparticles, either alone or together with Tb(dbm)3Phen, has a positive effect on luminescence, since the emission increases linearly up to much higher Eu-loadings (23 wt%) than in other matrices before showing concentration quenching. Co-doping with Tb enhances the emission of the material, but reaches a limit for a total amount of the complex of about 10 wt%. The results could be interpreted by using a simple sphere of action model

Acknowledgements

Alessia Le Donne, University of Milano Bicocca, is acknowledged for fruitful discussions.

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